The invention relates to a process for preparing a flexible graphite article that is embossed with a particular pattern thereon, and the article prepared thereby. By the practice of the present invention, a material is provided which consists of an article (such as a sheet) of flexible graphite having an embossed pattern thereon. Uses of the inventive material include as a component in an electrochemical fuel cell.
An ion exchange membrane fuel cell, more specifically a proton exchange membrane (PEM) fuel cell, produces electricity through the chemical reaction of hydrogen and oxygen in the air. Within the fuel cell, electrodes denoted as anode and cathode surround a polymer electrolyte to form what is generally referred to as a membrane electrode assembly, or MEA. Oftentimes, the electrodes also function as the gas diffusion layer (or GDL) of the fuel cell. A catalyst material stimulates hydrogen molecules to split into hydrogen atoms and then, at the membrane, the atoms each split into a proton and an electron. The electrons are utilized as electrical energy. The protons migrate through the electrolyte and combine with oxygen and electrons to form water.
A PEM fuel cell is advantageously formed of a membrane electrode assembly sandwiched between two graphite flow field plates. Conventionally, the membrane electrode assembly consists of random-oriented carbon fiber paper electrodes (anode and cathode) with a thin layer of a catalyst material, particularly platinum or a platinum group metal coated on isotropic carbon particles, such as lamp black, bonded to either side of a proton exchange membrane disposed between the electrodes. In operation, hydrogen flows through channels in one of the flow field plates to the anode, where the catalyst promotes its separation into hydrogen atoms and thereafter into protons that pass through the membrane and electrons that flow through an external load. Air flows through the channels in the other flow field plate to the cathode, where the oxygen in the air is separated into oxygen atoms, which joins with the protons through the proton exchange membrane and the electrons through the circuit, and combine to form water. Since the membrane is an insulator, the electrons travel through an external circuit in which the electricity is utilized, and join with protons at the cathode. An air stream on the cathode side is one mechanism by which the water formed by combination of the hydrogen and oxygen is removed. Combinations of such fuel cells are used in a fuel cell stack to provide the desired voltage.
One limiting factor to the use of flexible graphite materials as components for PEM fuel cells is the definition of a pattern embossed on the material, which, if not sufficient, can interfere with operation of the fuel cell, by either permitting leaking of fluids, or not permitting sufficient fluid flow through the fuel cell.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites by definition possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be chemically treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been chemically or thermally expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like (typically referred to as xe2x80x9cflexible graphitexe2x80x9d). The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is as much as about 80 times or more the original xe2x80x9ccxe2x80x9d direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material is typically within the range of from about 0.04 g/cc to about 1.4 g/cc. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the xe2x80x9caxe2x80x9d directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude typically, for the xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d directions.
This considerable difference in properties, i.e. anisotropy, which is directionally dependent, can be disadvantageous in some applications. For example, in gasket applications where flexible graphite sheet is used as the gasket material and in use is held tightly between metal surfaces, the diffusion of fluid, e.g. gases or liquids, occurs more readily parallel to and between the major surfaces of the flexible graphite sheet. It would, in most instances, provide for greater gasket performance, if the resistance to fluid flow parallel to the major surfaces of the graphite sheet (xe2x80x9caxe2x80x9d direction) were increased, even at the expense of reduced resistance to fluid diffusion flow transverse to the major faces of the graphite sheet (xe2x80x9ccxe2x80x9d direction). With respect to electrical properties, the resistivity of anisotropic flexible graphite sheet is high in the direction transverse to the major surfaces (xe2x80x9ccxe2x80x9d direction) of the flexible graphite sheet, and substantially less in the direction parallel to the major faces of the flexible graphite sheet (xe2x80x9caxe2x80x9d direction). In applications such as electrodes for fuel cells, it would be of advantage if the electrical resistance transverse to the major surfaces of the flexible graphite sheet (xe2x80x9ccxe2x80x9d direction) were decreased, even at the expense of an increase in electrical resistivity in the direction parallel to the major faces of the flexible graphite sheet (xe2x80x9caxe2x80x9d direction).
With respect to thermal properties, the thermal conductivity of a flexible graphite sheet in a direction parallel to the major surfaces of the flexible graphite sheet is relatively high, while it is relatively low in the xe2x80x9ccxe2x80x9d direction transverse to the major surfaces.
Flexible graphite sheet can also be provided with channels, which are preferably smooth-sided, and which pass between the parallel, opposed surfaces of the flexible graphite sheet and are separated by walls of compressed expanded graphite. When the flexible graphite sheet functions as an electrode in an electrochemical fuel cell, it is placed so as to abut the ion exchange membrane, so that the xe2x80x9ctopsxe2x80x9d of the walls of the flexible graphite sheet abut the ion exchange membrane.
A process permitting the embossing of a high definition pattern on a flexible graphite sheet, to provide a material uniquely capable of use as a component in a PEM fuel cell, is addressed by the present invention.
The present invention provides a material suitable for use in a PEM fuel cell. The material is formed of a sheet of a compressed mass of expanded graphite particles having a pattern embossed thereon. The pattern is advantageously formed by mechanically impacting an opposed surface of the graphite sheet to displace graphite within the sheet at predetermined locations to provide a channel pattern. The transverse fluid channels are separated by walls (the tops of which are often referred to as lands) of compressed expanded graphite.
The inventive process involves providing an embossing apparatus which generally comprises two opposed elements, one of the two opposed elements comprising an embossing element having an embossing pattern thereon, the embossing pattern formed by arraying a series of walls having lands (i.e., wall tops) which are a predetermined height from the surface of the embossing element and channel floors, about the embossing element; and the other of the two opposed elements comprising a landing element having an impact surface, wherein the embossing element and landing element are arrayed in the embossing apparatus such that the impact surface of the landing element is separated from the channel floors of the embossing element by a distance xe2x80x9cdxe2x80x9d which is at least equal to (and preferably greater than) the height of the lands; embossing a flexible graphite sheet by passing it between the embossing element and the landing element of the embossing apparatus such that the walls of the embossing element exert pressure on the flexible graphite sheet, wherein the flexible graphite sheet has a thickness in the region of the embossing pattern prior to embossing which is less than distance xe2x80x9cdxe2x80x9d, but greater than the distance between the impact surface of the landing element and the lands of the embossing element, thus forming a gap between the flexible graphite sheet and the channel floors of the embossing element, further wherein embossing of the flexible graphite sheet in the embossing apparatus causes material to flow from the area of the flexible graphite sheet which encounters pressure from the lands of the embossing element to the gap between the flexible graphite sheet and the channel floors of the embossing element.
The flexible graphite sheet is preferably impregnated with resin, such as an acrylic-, epoxy- or phenolic-based resin system, prior to embossing, and advantageously the resin is cured after the flexible graphite sheet is embossed. The resin content of the resin-impregnated flexible graphite sheet is preferably at least about 5%, and more preferably at least about 10%, by weight.
Moreover, in another preferred embodiment, porosity of the flexible graphite sheet is adjusted prior to embossing, to control the morphology of the finished embossed article. For instance, the sheet can be prepared such that it is relatively void-free. This can be accomplished, for instance, by calendering or pressing the flexible graphite sheet prior to embossing.